Earths at outbuildings...

[Boberto]

As ive always understood it if you were to run a SWA to a garage / shed, lets say 30m away, and if the earthing system at the origin (house) is tncs then a earth rod would need to be installed at the outbuilding because tncs/pme cannot be exported.

Where the earthing system at the origin is a tt system am I right in saying that an earth rod can be installed at the out building especially if Ze readings on the rod at the house are particularly high. If this is the case would a 2 core cable suffice as a supply to the outbuilding or would a 3rd core (earth) be advisable (belt and braces)???

What is the norm with a tns system? Export the earth or install earth electrode at outbuilding?

Ive just read this back to myself and I havent worded it brilliantly, and its late, and i've been on the Coors so apologies for being a thicko.

 

[Sintra]

There is nothing wrong with exporting PME / TNC-S especially if there are no extraeneous conductive parts in the out building.

 

[MarkieSparkie]

Hi Boberto, Not being able to export a TN-C-S (PME) earth is a widely accepted myth and the earthing of outbuildings is one the most frequent subjects on this forum. You really need to read the following sticky-thread which covers exported earthing in detail.
http://www.electriciansforums.net/e...159-good-guide-exported-pme-outbuildings.html

 

[Engineer54]

As ive always understood it if you were to run a SWA to a garage / shed, lets say 30m away, and if the earthing system at the origin (house) is tncs then a earth rod would need to be installed at the outbuilding because tncs/pme cannot be exported.

Where the earthing system at the origin is a tt system am I right in saying that an earth rod can be installed at the out building especially if Ze readings on the rod at the house are particularly high. If this is the case would a 2 core cable suffice as a supply to the outbuilding or would a 3rd core (earth) be advisable (belt and braces)???

What is the norm with a tns system? Export the earth or install earth electrode at outbuilding?

Ive just read this back to myself and I havent worded it brilliantly, and its late, and i've been on the Coors so apologies for being a thicko.

You should always run a CPC with your TT supplied sub-main cable. Not only that, you should also link the separate earth electrodes. Having said that, it doesn't matter too much if all you ever intend installing is a single 3/8'' 1.2m rod for your TT system!!! Your overall Ra value will still remain crap, and totally worthless...

 

[Deleted member 9648]

You should always run a CPC with your TT supplied sub-main cable. Not only that, you should also link the separate earth electrodes. Having said that, it doesn't matter too much if all you ever intend installing is a single 3/8'' 1.2m rod for your TT system!!! Your overall Ra value will still remain crap, and totally worthless...

In which case there are vast numbers of crap and worthless installs in the UK which nevertheless comply with Bs7671.

 

[Engineer54]

And you would be exactly right .....in both cases unfortunately. Maybe because BS7671 has the lowest standard of TT earth electrode requirements in Western Europe..

 

[Deleted member 9648]

I wont argue with your knowledge of worldwide systems E54,but would point out that in my neck of the woods there are many,many single rod TT systems with Ra values typically around 100+ ohms. I maintain and have installed many of them ,and can honestly say I have never known or heard of a problem which could be attributed to the Ra value,and which a lower value would have prevented.
While we have long agreed to disagree on this,where I see posts rubbishing standard UK TT practices which I believe are perfectly safe,I will always endeavour to put across the other side for a balanced view.

 

[Engineer54]

That's why we have to agree to disagree wirepuller, The Ra values you state here are high in their own right. If you then add to that, the very real fact that that during the freezing winters you seem to now get every year, those 100 ohm Ra values will literary sore, on a single 1.2 m to next to nothing. Likewise, if you ever get an extended heatwave, where the ground could well dry out , leaving that poor excuse for a earth electrode (1.2m/3/8'') providing very little in terms of an Ra value....

I have nothing against single earth electrodes, so long as they go DEEP!! Now be honest, when was the last time you have seen a modern TT system in your area, or the UK, that used more than one extendable earth electrode on a domestic installation?? Going from this and other Forums on this subject, your answers going to be very few to None!!

 

[diditrain]

If this is such an issue, would it not be written somewhere that one rod is going to be completely useless when the ground freezes

 

[Engineer54]

If this is such an issue, would it not be written somewhere that one rod is going to be completely useless when the ground freezes

It is a know fact, that frozen ground soils will represent very high resistances to earth electrodes and is highlighted to one extent or another in every publication covering this subject that i know of. I'm sure you'll also find this mentioned in the PDF file that Lenny posted in the sticky section of this forum, ....it'll be there somewhere guaranteed!!! ...lol!!

EDIT: It's around the the first metre from FGL that will be affected, and this is the length that will be attributing very little in the way lowing resistance. only the lower 20 cm will be working relatively normally...

 

[diditrain]

So a specified minimum depth may be appropriate then

 

[La Poste]

What sort of depth are we talking about, just a ball park figure?

 

[Engineer54]

So a specified minimum depth may be appropriate then

Basically, i think 1.2 metre rods should be relegated to use as the last extendable section of a deep driven rod installation. and the 1.2m 3/8'' taken out of the market place altogether....

Very difficult to stipulate a minimum depth, due to the numourous types of ground conditions you can encounter. Much better to have a realistic minimum Ra level, one that would exclude the use 1.2m rods... lol!!

 

[La Poste]

I didn't realise frost affected soil resistance readings so much until reading this and a few other threads.

The deeper the better then.

Let's just hope nothing is underneath to drive your rod into like a big underground power cable. I mean how do you know what's down there when you really want to put a deep spike in?

 

[Engineer54]

I didn't realise frost affected soil resistance readings so much until reading this and a few other threads.

The deeper the better then.

Let's just hope nothing is underneath to drive your rod into like a big underground power cable. I mean how do you know what's down there when you really want to put a deep spike in?

You'll know all about THAT probably before you've gone the first metre!! lol!! Water and gas pipes however are a completely different kettle of fish!! lol!! But the chances of you hitting them with the small rod diameter are quite slim. Far more chance of hitting a bloody big rock or old builders rubble!! lol!!!